The present invention relates to magnetic tape devices for recording data on a magnetic tape (hereinafter simply referred to as a "tape") or for reproducing data from the tape and, more particularly, to a magnetic tape device which performs a positioning operation by driving a magnetoelectric conversion element (hereinafter referred to as a "head core") in the direction of the width of the tape.
Such magnetic tape devices are used as an auxiliary storage device for computers and, in addition, as an auxiliary storage device for backing up magnetic disc devices, which are also used as an auxiliary storage device. In such magnetic tape devices, data is managed on the basis of a plurality of tracks, which are band-shaped data storage areas provided in parallel in the longitudinal direction of a tape, and blocks, which are strip-shaped data storage areas defined by dividing the tracks into pieces having a predetermined length in the longitudinal direction thereof.
For example, known conventional magnetic tape devices include fixed head type devices, such as the magnetic tape device Model 3490 from IBM Corp., U.S.A., wherein a multiplicity of head cores are arranged in parallel in the direction of the width of a tape to read and write the same number of data tracks simultaneously.
In this type of device, when the number of tracks per unit length in the direction of the width of a tape (track density) is increased to increase the storage capacity of the tape, the intervals between the adjoining head cores for reading the tracks must be decreased. The problem of interference between the adjoining head cores sets a limit to the decrease of the head core intervals. It is therefore not possible to decrease the track intervals below the head core intervals.
Serpentine type magnetic tape devices are known wherein a head, having a multiplicity of head cores arranged in parallel in the direction of the width a tape, is moved in the direction of the width of the tape to solve this problem. The operation of such devices will be described with reference to FIG. 15.
First, as shown in FIG. 15, a plurality of head cores 14 are arranged at certain intervals in the direction of the width of a tape on a head 10. When a tape 3 travels in a forward direction 59 from its position where the head 10 contacts the beginning of the tape (BOT) 55, the head cores 14 record or read a first group of tracks 57. When the head 10 is brought into contact with the end of tape (EOT) 56, the head 10 is moved in a tape width direction 61. When the head cores 14 come to a position where they directly face a second group of tracks 58, the tape 3 is moved in a reverse direction 60 to record or read the second group of tracks 58. By increasing the number of such groups of tracks, recording and reading can be performed on more tracks without increasing the number of the head cores.
In such a device, information is continuously recorded, for example, by recording it on the first group of tracks from the beginning of the tape and, after the end of the tape is reached, recording it on the second group of tracks.
On the other hand, to read information, the tape is moved to position the head at an end of the tape when an instruction is given to indicate the track (target track) and the block (target block) where the information to be read is present. Next, positioning is performed to cause the head cores to face the target track having the data to read at the end of the tape, and then the tape is moved. Then, reading is performed when the target block comes to the head.
Such an operation to move the head cores from the track which currently faces the head cores to a position where they face the track specified for the next recording or reading operation (hereinafter referred to as a "target track") is referred to as a "track switching" operation. The operation of moving the head in the direction of the width of the tape for such a purpose is referred to as "head positioning".
Known conventional methods for positioning a head include the open loop control system employed in the device disclosed in Japanese unexamined patent publication No. S61-182621 wherein positioning is performed by driving the head using a stepping motor for movement by an amount corresponding to a predetermined number of pulses, and the closed loop control system employed in the device disclosed in Japanese unexamined patent publication No. H4-209310, wherein a servo track on the tape is used for such a purpose.
The open loop control system employed in most conventional devices does not monitor the actual position of the tape. Therefore, this system is disadvantageous not only in that it can not follow fluctuations of the positions of tracks caused by the meandering of the tape during its travel in devices having a high track density, but also in that it can not properly handle a positional shift of a tape which has been loaded.
In devices utilizing a servo track, a servo track having a predetermined signal recorded thereon is provided in parallel with data tracks and the signal is read by a servo head core (hereinafter also referred to as a "servo head") which faces the servo track. Positional deviation between the servo head and servo track is obtained from the magnitude of the output of the servo head.
Referring to the method for obtaining such a positional deviation, for example, when the servo head is in a position where it can read the entire width of the servo track, the output becomes maximum, which means that accurate positioning has been achieved. A shift of the servo track from such a position results in a decrease in the output because only a part of the width of the servo track can be read. Since the amount of the decrease in the output is in one-to-one correspondence with the positional deviation between the servo track and servo head, the detection of the output will give the positional deviation.
Referring to FIG. 15, by providing a servo head core 12 and head cores 14 for reading data on a single head 10, so that the servo head core 12 is positioned on a servo track 11 with the head cores 14 being positioned on a group of data tracks 57, the head cores 14 can be positioned on the group of data tracks 57 through the positioning the servo head 12.
If there are a plurality of groups of data tracks on which a head must be positioned, the same number of servo tracks are usually provided as the number of groups of data tracks, and a track switching operation is carried out by positioning them on the respective groups of data tracks in accordance with the above-described configuration of the head. Although the simplest configuration has been briefly described, there are various known methods for recording and processing signals on a servo track which basically obtain the positional deviation between the track and head core.
Further, in the case of devices capable of detecting the deviation between the positions of tracks on a tape and a head using a servo track or the like, Japanese unexamined patent publication No. H5-46961 indicates that the following operation of the head can be performed so as to maximize the signal output in data read write operation modes.
Although there are data and servo tracks, as described above, hereinafter the term "track" means a data track.
In serpentine type magnetic tape devices represented by the so-called QIC (quarter inch cartridge) type, as described above, the tape is dragged in the moving direction of the head due to friction between the surface of the head and the tape itself as a result of the track switching operation of the head, when the tape is stopped or moved at a low speed.
In conventional devices, however, it is some time before a target block is accessed after track switching because track switching is performed when the tape has been wound around the reel on either side almost entirely, i.e., at the beginning or end of the tape. This allows a shift of the tape to be naturally corrected to some degree as a result of the movement of the tape, and the time required for such correction has caused no problem. Furthermore, in conventional devices wherein the track density is low, even if some tape shift remains uncorrected, it creates no problem during data recording and reading operations.
In high density devices, however, recording and reading operations must be performed on any track of any block on a magnetic tape. In this case, it must be possible to perform the track switching operation in any position over the entire length of the tape. However, it is undesirable for such a tape shift, during head positioning, to prolong the time before the commencement of the recording and reading of information.
In addition, in conventional devices utilizing a servo track, although the relative positional deviation between the head core and track can be obtained, it is not possible to know where the head is located in the movable range of the head at this time. Specifically, information is not available on where the head is located in its movable range on an axis along the moving direction thereof. Therefore, the positioning operation using a servo track does not work when the tape moving speed is low or when the servo signal can not be read as a result of a decrease in the signal to noise ratio. This results in a problem in that the time before the recording and reading operations is prolonged because the positioning operation must be performed after the servo track becomes readable.
Such head position information can be regarded as the absolute position of the head, if it is defined as displacement from a reference position defined on the above-mentioned axis, and will therefore be referred to as the "absolute head position" to be differentiated from a deviated position.